Today is the first day of the rest of Ashton Kutcher's life . . . with Mila Kunis. As Us Weekly confirmed late Thursday, Feb. 27, the couple of nearly two years are ready to make it official, and Kunis, 30, flashed an unmistakable diamond ring while shopping at the Grove in L.A. earlier that day.

"They are engaged," a source tells Us. "The ring is tasteful," adds the source, who estimates that the simple rock is around 5 to 6 carats. The Two and a Half Men actor proposed about a month ago, with Kunis wearing her new jewelry "out and about." The proposal was "very sincere and simple," the source reveals, adding that the low-key pair simply told family and a few close friends. And they're eager to exchange their vows, too: "It won't be a long engagement."

"He wants to marry her soon and start their life together," concurs a close Kutcher confidante. "He was planning this all along." The former That 70s Show costars began dating in the spring of 2012. The main obstacle that prevented Kutcher from proposing earlier? His protracted divorce proceedings with Demi Moore; he and the Margin Call actress, 51, split bitterly in fall 2011 but didn't finalize their divorce until last November.

"This was the plan," the Kutcher source adds. "Once the divorce was final he got it ready and proposed. He knew through most of the time he was trying to get the divorce over with he would propose as soon as it was over."

Kutcher was by his wife-to-be's side during a Feb. 19 event at Bill's Food and Drink in NYC for Jim Beam, the bourbon brand which recently named Kunis its ambassador. As the Ted actress (sans ring) unveiled her ads and sipped custom Jim Beam cocktails, Kutcher "was reacting to the ads and laughing at her jokes, looking adoringly at her," an observer says.